A second explosion has hit Fukushima nuclear plant sparking fresh fears of a
reactor meltdown.

Shortly after Prime Minister Naoto Kan appeared on television to inform the nation that the plant was in an "alarming" state, its No. 3 reactor erupted in a resounding blast that was felt 25 miles away.

Six workers, and four soldiers were injured in the explosion that sent clouds of white smoke billowing into the sky.

Scientists and technicians have been battling to control two overheating reactors at the 40-year-old Fukushima Daiichi facility after cooling systems were knocked out by Friday's massive 9.0-magnitude quake.

Japan's government has sought to play down fears of a dangerous radiation leak, saying the reactor's inner containment vessel, which holds the nuclear fuel rods, was still intact following the blast, which was caused by a hydrogen build up.